The term Big Four originates in accounting where there actually are a specific set of four large, established companies.

For tech, I usually remember it as AmaGooBookSoft - Amazon, Google, Facebook and Microsoft. Apple is sort of a hardware company so sometimes it is included and sometimes it is not.

I've always heard it as Google, Facebook, Apple and Amazon. Microsoft isn't very sexy to the typical developers that participate in the startup/Silicon Valley "scene" and its accompanying communities (HNews included).

Big four almost always includes google and facebook but the other two slots seem to vary a bit based on who is saying it. I've even seen companies like yahoo and twitter thrown in there.